1. Field of the Invention
The present invention relates to a motor, particularly, relates to a motor mounted with an improved dynamic pressure fluid bearing mechanism.
2. Description of the Related Art
It has been common to a motor mounted with a dynamic pressure fluid bearing in the past that the motor was constituted by a pair of radial dynamic pressure fluid bearings provided in respective positions isolated from each other in the axial direction and a taper seal section for sealing lubrication fluid in the dynamic pressure fluid bearings, wherein the pair of radial dynamic pressure fluid bearings and the taper seal section were disposed so as to be in series in the axial direction.
Accordingly, there existed a problem such that a total thickness of the motor or a total length of the motor in the axial direction was hardly thinned or shortened. In order to solve the problem, the Japanese publication of unexamined patent applications No. 2004-270820 disclosed a motor.
The motor disclosed in the Japanese publication of unexamined patent applications No. 2004-270820 is constituted as mentioned below (see FIG. 1 in the publication No. 2004-270820).
More specifically, in the motor, a radial dynamic pressure fluid bearing is constituted by a shaft and a sleeve, which supports the shaft.
Further, one pair of radial dynamic pressure fluid bearings is provided in isolated positions from each other in the axial direction.
Furthermore, either one surface of a top end surface of the sleeve and a bottom end surface of a hub, which confront with each other, is provided with a thrust dynamic pressure groove for generating dynamic pressure, and resulting in constituting a thrust dynamic pressure fluid bearing. The thrust dynamic pressure groove generates dynamic pressure or force directed toward a direction of so-called “pump-in” with respect to lubrication fluid when a rotor of the motor rotates. The force in the “pump-in” direction acts on the lubrication fluid and increases pressure of the lubrication fluid totally.
Accordingly, first force acts on a hub and a motor base respectively in a direction of separating the hub and the motor base from each other when the rotor of the motor rotates.
On the other hand, a yoke in an annular shape made from soft magnetic material is disposed on a motor base, and a magnet in a ring shape is provided on the hub so as to confront with the yoke. By suction power that interacts between the yoke and the magnet, second force, which directs the hub and the motor base toward a direction of approaching with respect to each other, is applied to the hub and the motor base so as to oppose the first force.
Accordingly, by balancing the first force and the second force, the rotor of the motor is made to rise and rotate with respect to the stator.
In the motor constituted by the above-mentioned configurations, a taper seal section is disposed outside the sleeve. In other words, the taper seal section is disposed in parallel with the radial dynamic pressure fluid bearing in the axial direction not in series with the radial dynamic pressure fluid bearing.
Accordingly, a length of the motor in the axial direction results in being able to be shortened.
Further, the sleeve is provided with a through hole that is bored in the axial direction. The through hole links both outermost sides of the pair of radial dynamic pressure fluid bearings in the axial direction. Consequently, pressure of the lubrication fluid propagates through the through hole and the pressure is averaged within the radial dynamic pressure fluid bearings in total, even though pressure of the radial dynamic pressure fluid bearings is likely to be unbalanced by disturbance.
Accordingly, the pressure balance is never disturbed in the motor.
With paying attention to force or pressure in the thrust direction that is generated in the conventional motor, force, which directs the hub toward a direction of leaving from the motor base or an elongation direction, depends only on pressure of lubrication fluid, wherein the pressure of lubrication fluid is increased by the thrust dynamic pressure groove.
The pressure is weakened by loss, which occurs when pressure of the lubrication fluid propagates through the through hole, in comparison with a case of excluding such a through hole. In the case of the conventional motor, the through hole is formed so as to link both end sides of the sleeve. Therefore, a length of the through hole is relatively long, and the loss is relatively large.
Accordingly, it is necessary for the dynamic pressure groove to be formed larger so as to obtain prescribed force. As a result, dimensions of the motor are likely to be enlarged in some cases.
Further, action of the force differs from directions, that is, the approaching direction or the elongation direction, and following differences in characteristics may occur.
Rising pressure of the lubrication fluid caused by the thrust dynamic pressure groove depends on a rotational speed of the rotor. However, suction power of the magnet is independent of a rotational speed of the rotor.
Accordingly, it is hard to balance force that raises the rotor in response to a rotational speed of the rotor. In the case of manufacturing various kinds of motors of which a rotational speed differs from each other, each motor must be independently designed for appropriate performance.
Furthermore, the narrower a gap in a thrust dynamic pressure fluid bearing is, the more repulsive force or force of separating from each other that is generated by the thrust dynamic pressure groove increases. The narrower a gap between the magnet and the yoke is, the more suction power of the magnet increases.
In other words, as the hub approaches the motor base more, repulsive force between the hub and the motor base increases, and as the hub gets away from the motor base, the repulsive force weakens.
Accordingly, respective force in opposite directions, which are generated respectively in response to a degree of the gap in the thrust dynamic pressure fluid bearing, show a similar tendency to increase or decrease, so that it is hard to balance the respective force in the opposite directions. Therefore, it is hard for the respective force to be designed for optimum conditions.
In addition thereto, the through hole provided on the sleeve links the both end portions of the sleeve and must be pierced by a narrow orifice over a relatively long distance. Consequently, a drill bit that is small in diameter and long in length is necessary for a process to make the through hole. However, such a drill bit is easy to break. In order to make a through hole being small in diameter without breaking such a drill bit, boring the through hole must be performed at a reasonable processing speed.
Accordingly, productivity is hardly improved by increasing a processing speed.